You need to give us some more info--- do you want a conventional airgapped output and\or would you consider a parafeed strategy?
Your probably fimiliar with the yahoo group (think it's on yahoogroups) dedicated to the GM 70 tube. This might be a resource to check out if your not already fimiliar with this group.
If going parafeed like Dowdy--- Jim uses our EXO-050 (5K nom pri impedance).
We may have some coil stock (really buried) of our FS-007 which had a nom pri impedance of 6,000 ohms. It could be airgapped for 100 mils.
Trick is that at 100 mils and a relatively large pri impedance--- it's going to get tough to get the optimal L with the relatively high magnitude of DC current through the primary winding. In several ways an airgapped OT along these parameters is going to have to sacrifice one for the other.
There is no beating physics :=)
That said--- just for kicks I did some preliminary design work on an airgapped unit that could carry 400 vrms at 30hertz along with 100 mils of dc plate current while developing a calculated 43 henries of inductance.
Overall--- it didn't look to bad on paper. Had a very nice relative perm--- reasonably low dcr (for pri)of 175 ohms or so--- and was not outlandishly large (which kills off high freq all other things being equal). Not ideal--- and rarely do I find designs that carry larger amounts of unbal dc plate currents able to provide the inductive stability that I aim for. This would be a custom design. With several winding patterns and strategies depending on the health of your pocketbook.
I could also potentially custom design you a parafeed output for 7K nom pri impedance---- would be smaller, have tons more L, operate more conservatively (flux density wise), and etc.
choices, choices, choices :=)
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MSL
Builder of MagneQuest™ & Peerless™ transformers since 1989
